For mice, the problem with using Gr-1 and CD11b is that monocytes and neutrophils will express both of these markers, and they are both present in the spleen. Typically, the mechanism of suppression by MDSCs is either through iNOS or Arginase I. You should probably stain for one of these intracellular proteins also. But, activated monocytes/macrophages can express these too! I think it's difficult to say that a cell is a true MDSC without doing a functional assay. Perhaps there has been some other recently described markers that are more specific.
In human, you can analize on CD14 (monocytes) and CD15 (granulocytes)cells the expression of IL4Ra and HLA-DR (lower/negative expression are consider "MDSCs"). That is only a phenotypical characterization to detetect "MDSCs-like cells". Suppressive function of MDSCs are releted to iNOS and arginase activity. As Nuruddeen said, you have to perform a functional assay to be sure that are really suppressive. Moreover, some paper define MDCSs CD14 or CD15 expressing IL-10 and/or TGFbeta.